It is difficult to directly measure the charging level (SOC; State Of Charge) of an electricity storage device, that is, the available battery capacity. However, it is known that there is some amount of correlation between the SOC of the electricity storage device and an open voltage (open-circuit voltage). Hence, conventional technologies obtain an open voltage through a measurement and a calculation, and obtain the SOC based on the correlation between the open voltage (internal voltage) and the SOC.
When the open voltage is E, and the voltage and current of the electricity storage device are V and I, the following formula (1) can be obtained.E=V+I·r  (1)
where r is an internal resistance of the electricity storage device. In the formula (1), for the internal resistance r, for example, a constant value (r0, a value when, for example, the SOC is 10% and a battery temperature is 20° C.) is applied.
However, the internal resistance r varies depending on the battery temperature and the SOC at any given point in time. Hence, a method of correcting the internal resistance r using the following formula (2) is disclosed (see, for example, Patent Literature 1).r=r0·A2·A1  (2)
where r0 is a predetermined resistance value given in advance for a battery. A1 is a first resistance ratio based on a battery temperature T. In addition, A2 is a second resistance ratio based on a predetermined reference charging level.
As is indicated above, it is apparent that there is a correlation between the SOC and the internal resistance. Hence, an available battery capacity detection device is disclosed that determines a deterioration level of a battery based on a resistance value dominated by internal mass transfer in the battery that is one of the internal resistances and has a high correlation with the available battery capacity (see, for example, Patent Literature 2). This available battery capacity detection device performs discharging with the battery being connected to a load resistor through a switch, and calculates a resistance value dominated by internal mass transfer in the battery based on a voltage across the terminals of the battery and a discharge current.
Conversely, a battery detection unit is disclosed that performs on/off switching at a predetermined frequency by a switch to discharge a battery that is connected to a load resistor, and that detects a voltage across the terminals and a current of a discharge circuit, thereby calculating the internal impedance of the battery (see, for example, Patent Literature 3).
Still further, an available capacity equalization device is disclosed that equalizes the variation in the SOC of electricity storage devices when multiple battery cells are connected in series and are loaded as a high-voltage battery (see, for example, Patent Literature 4). This available capacity equalization device sets, for each cell, a bypass circuit that includes a voltage sensor, a bypass resistor, and a bypass switch, and also controls this bypass switch.
More specifically, this available capacity equalization device causes a pre-installed load resistor to discharge to a target predetermined voltage at a timing for deactivating the operation of the battery storage device for a battery cell with a high voltage to equalize the voltages of the multiple connected batteries, for example. This causes the respective available capacities to be equalized among the battery cells.
The above-explained available battery capacity detection device, battery detection unit, and available capacity equalization device are all discharge devices that cause the electricity storage device to discharge to the load resistor in order to measure the condition of the battery, and to equalize the variation in the respective SOC of the electricity storage devices.